1. Technical Field
The present invention relates to a three-phase voltage source inverter system.
2. Related Art
A multistage-multiplex method has been known as a method for improving the output voltage waveforms of a three-phase inverter. For example, a three-phase inverter system having a configuration where three three-phase inverter units INV-1 through INV-3 are connected to a direct-current power source Ed by a three-stage cascade arrangement; Δ windings 101-1 through 101-3 in three-phase transformers T1-T3 are connected to three phases U, V, W of these three three-phase inverter units INV-1 through INV-3; and Y windings 102-1 through 102-3 in three-phase transformers T1-T3 respectively are the outputs of the U, V, W phases, as shown in FIG. 4, is used in order to obtain voltage having a staircase waveform consisting of 18 steps per cycle.
In this existing three-phase inverter system, the numbers of turns of the Δ windings 101-1 through 101-3 in the three-phase transformers T1-T3 utilized for phase shifts are made uniform respectively; the turn ratio of the Y windings 102-1 through 102-3 are set as A:B:C=1:0.742:0.395; the three-phase inverter units INV-1 through INV-3 are sequentially controlled with a phase difference of 20° delay; and then the output phase voltage vUN has a staircase waveform consisting of 18 steps per cycle, as shown in FIG. 5(a). Therefore, an output line-to-line voltage vUV can obtain a staircase waveform consisting of 18 steps per cycle, as shown in FIG. 5(b).
However, the existing three-phase inverter system has a problem that the more the output voltage waveform approaches a sinusoidal waveform, the more phase shift transformers and inverter units are needed, so a larger apparatus and cost increases cannot be avoided. For example, 18 switching elements and three three-phase transformers are needed to obtain the output voltage having 18 steps per cycle by a three-stage cascade arrangement shown in FIG. 4.
Meanwhile, for example, methods for improving a source current waveform by injecting higher harmonic waves into the source side of a three-phase rectifier circuit have been suggested in the research papers: “An Improved Three-Phase Diode Rectifier for Reducing AC Line Current Harmonics,” in Conf. Rec. EPE'97, pp. 4.227-4.232, written by S. Masukawa and S. Iida; “New Multiplication Technique Based on Six-Pulse Thyristor Converters for High-Power Applications,” IEEE Trans. Ind. Appl., Vol. 38, No. 1, pp. 131-136, 2002; written by Sewan Choi; and “New Passive and Hybrid Rectifier Topologies to Achieve Sinusoidal Utility Current and Controlled Output Voltage,” in Conf. Rec. IPEC-Niigata 2005, pp. 1823-1829,2005, written by Y. Nishida. These methods have been considered for the rectifier circuit for an aircraft generator or an uninterrupted power supply (UPS), since it can obtain the same improvement effect of waveforms as that in the multistage-multiplex method simply by adding an auxiliary circuit with a minimal capacity for injecting the higher harmonic waves. However, no previous case in which this method is applied in the improvement of the output voltage waveform of an inverter has been found.